1. Field of the Invention
The present invention relates to a ring access network for a mobile radiotelephony network. It proposes a novel architecture of the access network to improve the quality of the communication, whether in vocal or data form, during automatic intercellular transfers. This improvement in quality is obtained both by a reduction in the duration of the transfer itself and by the introduction of macrodiversity which enables the maintaining, for a certain time, of the mobile station in a link with the former base station and the new target base station. The information elements flowing through these two links are combined at a point of the network in order to provide a single flow of information of higher quality.
This invention may be implemented in a particularly promising way on two types of existing networks, namely networks meeting the DQDB (Distributed Queue Dual Bus) standard and those meeting the ATM (Asynchronous Transfer Mode) standard.
2. Description of the Prior Art
The aim of a GSM (Global Systems for Mobile Communications) type network where the transmission between the mobile station and the network is digital is that it provides telecommunications services to subscribers irrespectively of their movements within a service area. The mobile subscriber uses a mobile station, generally formed by a mobile piece of equipment, that gives the radio and software capacities needed for the dialog with the network. The presently used radiotelephony networks, such as the GSM network, have a centralized star architecture as can be seen in FIG. 1, comprising base stations (BTS or Base Transceiver Stations) that ensure the radio coverage of the service area and are each linked to a Base Station Controller (BSC). Each base station controller (BSC) is connected to a mobile services switching center (MSC) itself connected to the subscribers of the fixed network. Physically, the information and the signals flow between the mobile object and the mobile services switching center. The base station controller organizes the supervision, allocation and releasing of the radio channels for activities that it does not analyze, having no knowledge of the subscribers, their identities and their calls, while the mobile services switching center manages the setting up of the call, the release of the call and all that is related to the identities of the subscribers and their calls.
Assuming that there will be a rapid development of radiotelephony, it is necessary to envisage small-sized cells for high density zones, but the reduction of the size of the cells as compared with presently used systems calls for an improvement in the devices enabling automatic intercellular transfers because these transfers will become more frequent, an automatic intercellular transfer (AIT) enabling a mobile station to change its point of linkage to the land-based network while at the same time maintaining the call that is in progress.
This generally star-shaped architecture of radiotelephony networks makes it necessary to set up a new circuit from a fixed point up to a new base station. This means that it becomes necessary to implicate the entity or entities located between the fixed point and new the base station. This has the drawback of creating substantial time gaps, of more than 200 ms for the GSM system. These gaps are perceptible by the user or interrupt the transmission of data for a non-negligible period of time.
A second drawback of this star-shaped architecture is the impossibility of direct communication between the base stations or between the base station controllers, for it is obligatory to go through the higher hierarchical level, namely the base station controller or the mobile services switching center. This indirect communication unnecessarily burdens the processor managing this higher level and delays the arrival of the messages at the destination base station. These messages are used partly to control the intercellular transfers.
A third drawback is the impossibility of sharing the total passband among the different ends of the star should a link between a base station and the base station controller or between a BSC controller and an MSC switch be saturated while the neighboring link is free.
Finally, the introduction of macrodiversity, namely the simultaneous use of two or more radio links, makes it necessary for currently used radiotelephony systems to manage a particular unit of equipment in the network or the mobile station (MS) and procedures to activate, implement and stop the macrodiversity. This unit of macrodiversity equipment should manage, firstly, the two circuits, the old and the new, concerned by the call in progress, in the downward or upward direction from the mobile stations to the network and, secondly, the release of the old link when it becomes unnecessary. Furthermore, in the case of macrodiversity in the mobile station, it is necessary to possess or set up a second downward link that will convey the same information element from the network to the mobile stations as the one carried by the first downward link. Two links are thus used to convey the same information element.